1. Field of the Invention
The present invention relates to a magneto-optical recording/reproducing apparatus, and more particularly to an optical head for a magneto-optical recording/reproducing apparatus in which an overwrite and a verification immediately after recording can be performed simultaneously by a single laser beam source.
2. Related Background Art
As disclosed in Japanese Patent Laid-Open No. 51-107121, there has been proposed an overwritable magneto-optical disk which adopts a system to modulate a magnetic field applied to a magneto-optical recording medium in accordance with information to be recorded. Also, Japanese Patent Laid-Open No. 64-82348, by way of example, proposes an optical disk apparatus that, in addition to the above system, a plurality of beams for recording and verification are arranged over a track to permit all steps of erasing --recording--reproducing while a disk is making a turn, with an aim of improving data processing speed for magneto-optical disks.
The prior art will now be described with reference to FIGS. 1 and 2 In FIG. 1, denoted by 1 is a magneto-optical disk as a rotating recording carrier which has, on a disk-like transparent substrate 103, a magneto-optical recording medium (film) 101 having a magneto-optical effect and a protective film 102. Light emitted from a light source comprising a semiconductor laser 2, for example, is converted by a collimator lens 3 into a parallel beam and then by a beam shaping optical system 4 into a beam having a circular distribution of the light intensity. Subsequently, the light is separated by a diffraction grating 5 into a plurality of beams (i.e., three beams of 0th and .+-.1st orders) which enter a condensing lens 7 through a beam splitter 6. The light beams condensed by the lens 7 impinge upon the disk 1 from the side of the disk substrate 103 to form small spots of about 1 .mu.m diameter on the recording film 101. The condensing lens 7 is attached to an actuator 8 so that the focal point is always on the recording film following vertical deflections of the disk 1 and the spots are always on a desired track following eccentric offset of information recording tracks on the disk. The reflected light from the disk 1 passes through the condensing lens 7 and is reflected by the beam splitter 6 to be led to a signal detection optical system 9 for detecting a magneto-optical signal and optical spot control signals in relation to a focus deviation, a track deviation, etc.
FIG. 2 shows respective light spots on the recording film and the intensity of those light spots during periods of recording and reproduction. The following description will be made in connection with the case where three light spots are formed by the diffraction grating. A central spot SP.sub.2 corresponds to the diffracted light of 0th order, while SP.sub.1 and SP.sub.3 correspond to the diffracted light of .+-.1st order. Taking a disk rotating direction as shown, the light spots SP.sub.3, SP.sub.2, SP.sub.1 pass over some point on the disk in this order. Accordingly, it is possible to assign SP.sub.2 to a recording/erasing spot and SP.sub.1 to a reproduction spot for error check. The light intensity ratio of one spot to the other is set such that, during the period of recording, the reproduction spot has reproduction power and the recording/erasing spot has recording power. This setting can be optionally determined by changing the structure of the diffraction grating 5. When the reproduction power is 1 mW and the recording power is 7 mW, for example, it is only required for the light intensity ratio to be 1:7.
FIG. 2 also shows a radiation power of the laser beam source during the periods of reproduction and recording. During the reproduction period, the laser 2 is operated to emit a beam at low power P.sub.1. At this time, SP.sub.2 has the reproduction power to (a) reproduce a magneto-optical signal, (b) reproduce a signal of address information or the like in the case of disks that such information is previously formed in the form of rugged pits, (c) detect a focus deviation signal, and (d) detect a track deviation signal. SP.sub.1 is not used here because of low power.
Next, during the recording period, the laser 2 is operated to emit a beam at high power P.sub.w. At this time, SP.sub.2 serves as a light spot for recording/ erasing, i.e., overwrite. Upon irradiation of the laser beam at high power, the temperature of the recording film 101 is raised to lower a degree of magnetization and coercive force. On this occasion, when a magnetic field with its polarity inverted depending on the information to be recorded is applied by a magnetic head 10, the recording film 101 is fixedly magnetized, while being cooled, in the direction of the magnetic field applied. Since the previous information is erased upon a rise in the temperature of the recording film 101, it is possible to simultaneously erase old information and record new information, i.e., to perform an overwrite. During the recording period, SP.sub.2 also detects both the focus deviation signal and the track deviation signal. On the other hand, SP.sub.1 now has the reproduction power and thus serves to reproduce the magneto-optical signal for error check immediately after recording.
The signal detection optical system 9 will be next explained. In the illustrated prior art, the detection optical system comprising a .lambda./2 plate 901 and a polarizing beam splitter 903. The focus deviation signal is detected using a lens 902 and light detectors 904, 905 arranged at respective positions spaced forwardly and rearwardly of the focus point by the same distance, to thereby obtain the focus deviation signal from changes in size of the light spots on the light detectors. Further, the track deviation signal is detected by the so-called push-pull technique.
In the above prior art, however, because of using the diffraction grating 5 to create the recording/ erasing (overwrite) spot SP.sub.2 and the reproduction (verification) spot SP.sub.1 for error check, there has produced the spot SP.sub.3 which is not necessary for the specific purpose. Thus, the quantity of light emitted from the semiconductor laser 2 has been wasted in an amount assigned to the spot SP.sub.3, thereby requiring a semiconductor laser of higher output power and a collimator lens 3 of higher N.A. to compensate for the wasted light amount.
Moreover, an angle adjustment for placing the plural spots from the diffraction grating 5 on the same track has been time consuming and hence costly.
Due to manufacture errors in the diffraction grating 5, it has been also difficult to keep constant a light amount ratio of the overwrite spot SP.sub.2 to the verification spot SP.sub.1 and a spacing between those two spots on the track.
In addition, detecting the magneto-optical signal has required complex and expensive optical parts such as the .lambda./2 plate 901 and the polarizing beam splitter 903.